Materials with high atomic weights such as nuclear materials can be detected by various methods. One notable technology is muon tomography which exploits scattering of highly penetrating cosmic ray-produced muons to perform non-destructive inspection of the material without the use of artificial radiation. The Earth is continuously bombarded by energetic stable particles, mostly protons, coming from deep space. These particles interact with atoms in the upper atmosphere to produce showers of particles that include short-lived pions which decay producing longer-lived muons. Muons interact with matter primarily through the Coulomb force without nuclear interaction. Muons radiate energy much less readily than electrons and lose energy due to scattering through electromagnetic interactions. Consequently, many of the cosmic ray-produced muons arrive at the Earth's surface as highly penetrating charged radiation. The muon flux at sea level is about 1 muon per cm2 per minute.
Muon tomography utilizes cosmic ray-produced muons as probing particles and measures scattering of such muons that penetrate through a target object under inspection. As a muon moves through the material of the target object, Coulomb scattering off of the charges of sub-atomic particles perturb its trajectory. The total deflection depends on several material properties, but the dominant effect is the atomic number, Z, of nuclei. The trajectories are more strongly affected by materials that make good gamma ray shielding (such as lead and tungsten for example) and by special nuclear material (SNM), that is, uranium and plutonium, than by materials that make up more ordinary objects such as water, plastic, aluminum and steel. Each muon carries information about the objects that it has penetrated, and measurements of the scattering of multiple muons can be used to probe the properties of these objects. For example, a material with a high atomic number Z and a high density can be detected and identified when the material is located, inside low-Z and medium-Z matter.